1. Field of the Invention
The present invention relates to a scattered ray absorption grid, more particularly to a scattered ray absorption grid having a plurality of plate members for constituting a grid formed by use of powders made of a radiation absorption material.
2. Description of the Related Art
A scattered ray absorption grid has been heretofore known, which is disposed between a subject for photography and a radiation detector and obtains radiation with a high S/N ratio by absorbing a scattered ray scattered by the subject for photography when an image is photographed by a radiographic imaging device.
This scattered ray absorption grid is constituted by arranging a plurality of plate members at intervals, which constitutes a grid portion. Each of the plate members for constituting a grid is formed by a slender and thin plate made of a material absorbing radiation. The scattered ray absorption grid absorbs radiation which is scattered by the subject for photography and travels obliquely, and effectively transmits only radiation from a radiation source which is made to be incident directly onto the radiation detector through the subject for photography. Thus, noise due to the detected scattered radiation mixed into an image of the subject for photography are reduced.
A high radiation absorption property is required for the plate members constituting the grid portion. In other words, a material having a high density must be used for the plate members, and generally a material obtained by processing lead to a thin plate shape is used.
As such a material for the plate members for constituting a grid having a high density, tantalum (Ta) and tungsten (w) are preferable from the viewpoint of the radiation absorption property, and it is known that tungsten (W) has a particularly excellent radiation absorption property.
However, tungsten has a high degree of hardness and an extremely high melting point, and is difficult to process. Accordingly, it is difficult to process tungsten to a slender and thin plate member for the grid, and said processing, if performed, would be quite high in cost.
The present invention was made in consideration of the foregoing circumstances. The object of the present invention is to provide a scattered ray absorption grid which is low in cost and has excellent scattered ray absorption property.
A scattered ray absorption grid of the present invention is composed of a grid portion constituted by use of plate members formed in such a manner that powders containing tungsten 50% by weight or more are hardened with binder so that the powders show a spatial filling rate of 40% or more, more preferably 60% or more. Alternatively, the scattered ray absorption grid of the present invention is composed of a grid portion constituted by use of plate members formed in such a manner that, grid materials formed by hardening powders containing tungsten 50% by weight or more with binder so that the powders show a spatial filling rate of 40% or more, more preferably 60% or more, are arranged on a substrate.
Furthermore, the foregoing powder containing tungsten 50% by weight or more means the one containing tungsten 50% by weight or more regardless of an existence state of tungsten such as tungsten compound including tungsten alloy, and tungsten mixture in which tungsten and other substances are physically mixed. Specifically, for example, even powder formed of only calcium tungstate CaWO4 that is tungsten compound is included in the foregoing powder containing tungsten 50% by weight or more because this powder contains tungsten W 50% by weight or more. The foregoing powder containing tungsten 50% by weight or more includes powder formed of an alloy containing tungsten and other metals, for example, an alloy formed of tungsten W and lead Pb, if this powder contains tungsten 50% by weight or more. Furthermore, if powder formed of pure tungsten W and substance containing no tungsten contains tungsten 50% by weight or more, this powder is included in the foregoing powder containing tungsten 50% by weight or more, as a matter of course. In addition, if powder formed of tungsten compound, pure tungsten and substance containing no tungsten contains tungsten W 50% by weight or more, this powder is included in the foregoing powder containing tungsten 50% by weight or more.
As the tungsten compound, besides the foregoing calcium tungstate CaWO4, enumerated are, for example, iron tungstate FeWO4, lithium tungstate LiWO4, magnesium tungstate MgWO4, barium tungstate BaWO4, sodium tungstate Na2WO4, nickel tungstate NiWO4, lead tungstate PbWO4, tungsten boride W2B, WB and W2B5, tungsten carbide WC and W2C, tungsten oxide WO, W2O3, WO2 and W2O5, tungsten sulfide WS2 and WS3, tungsten silicide WSi2, WSi3 and W2Si3 and the like. As other metals forming the alloy together with the foregoing tungsten, enumerated are, for example, Co, Pt, Ni, Fe, Mo, Cr, Fe, Ti and the like in addition to the foregoing lead.
The foregoing binder should be an organic binder or a metal with a melting point less than the melting point of tungsten.
Furthermore, the binder, in the case that a body is formed by use of powder and the like as a main raw material, refers to a substance blended into the powder to maintain a shape of the body and to enhance the structural integrity thereof.
The aforementioned metal refers to those including alloys, and a metal showing a high density and an excellent radiation absorption property should be employed.
As the organic binder, for example, resin materials should be used so that particles constituting the powder are bound and the powder can maintain a stable shape.
It is not always necessary that the particles constituting the powder contain tungsten at a constant rate. Therefore, each of the particles can contain a different amount of tungsten as long as the powders as a whole contain a predetermined amount of tungsten.
A slender and thin plate extending in one direction should be used as the plate member for constituting a grid.
The inventor of the present invention made various investigations concerning the radiation absorption property of the plate member for constituting a grid, which was formed by hardening tungsten powder with a binder. As a result of these investigations, the inventor learned that a plate member for constituting a grid that shows an excellent radiation absorption property could be obtained when the amount of tungsten contained in the powder is set to 50% by weight or more and when a spatial filling rate in constituting the plate member by use of the powder is set to 40% or more, more preferably 60% or more. Based on this knowledge, the inventor arrived at the present invention.
According to the scattered ray absorption grid of the present invention, the grid portion is constituted by use of plate members formed by hardening powder made of tungsten, which is relatively low in cost and has an excellent radiation absorption property, with a binder, or alternatively the grid portion is constituted by use of plate members formed in such a manner that grid materials obtained by hardening the tungsten powders with the binder are arranged on a substrate. Therefore, the processing of the plate members for constituting a grid is very easy, and a productivity of the grid portion made of tungsten is enhanced. Accordingly, the scattered ray absorption grid can be obtained at low cost.
If an organic binder is used as the binder, it is possible to form the plate members for constituting a grid more easily, for example, by kneading the tungsten powders into a binder which was melted at a relatively low temperature and by molding the mixture of the tungsten powders and the binder. Accordingly, costs of the scattered ray absorption grid can be further reduced.
In the case where a metal having a melting point less than the melting point of tungsten is used as the binder, if lead which has an excellent radiation absorption property is, for example, used as the binder, the radiation absorption property of the plate members for constituting a grid obtained by hardening the tungsten powders with the binder can be further enhanced.